In the mobile communication system of the present generation (e.g., the third generation (3G)), a system configuration (a service area range) is built up extremely widely. The mobile station's being capable of migrating (moving) across between the respective service areas is an inevitable function to the mobile communication system. A process in which the mobile station migrates across between the service areas (the process of switching over a wireless base station to which the mobile station is wirelessly connected) is generally called a handover process. The handover process involves adopting a technology enabling a target mobile station to migrate across between the service areas so as to keep seamless communications between the handover target mobile station and the connected base station, in which an end user is provided with a stressless service with no disconnection of the communications.
In an LTE (Long Term Evolution) system also of which specifications are developed as one of candidates for the next generates mobile communication system, definitions of a handover function and a related control system are underway by way of one of seamless communication technologies as a concomitant of the migration of the mobile station across between the service areas (the development of the specifications is underway).
According to the handover specifications that are currently examined and developed in the LTE system, what is examined is a method of forwarding transmission data directed to the mobile station to a handover target wireless base station device from a handover source wireless base station device with respect to the migration of the mobile station across between the wireless base stations each configuring its service area (cell).
FIG. 1 is a diagram illustrating one example of configuration for the LTE system. As illustrated in FIG. 1, the LTE system includes a plurality of wireless base station devices (eNode-B (eNB)) and an MME/SAE gateway (MME/SAE Gateway) corresponding to a superior apparatus above the eNB) each holding the wireless service area.
The MME/SAE gateway is connected via an S1 interface to the eNB. Further, the eNBs (nodes) are connected to each other via an X2 interface.
The MME/SAE gateway handles termination of a NAS (Non-Access Stratum) signal, management of the mobile station (UE: User Equipment) in an idle status, management of an SAE bearer resource, and so on.
The eNB handles call control and wireless (radio frequency: RF) control, and has functions such as RRC (Radio Resource Control), an S1 interface access point (S1-AP) and an X2 interface access point (X2-AP).
[Non-patent document 1] TS 36.300 V8.0.0 (2007-03) Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Release 8)
[Patent document 1] Japanese Patent Laid-Open Publication No. 2007-53805
In the next generation mobile communication system, however, for effectively utilizing finite resources such as wireless resources (for improving frequency usage efficiency), there is a progress of development of specifications based on a premise of the shared channel allocation (the wireless CH (channel) is shared) without following logic of the individual channel resource allocation as adopted in the existing system such as W-CDMA.
FIG. 2 is an explanatory diagram illustrating one example of the shared channel allocation (one example of (OFDMA (Orthogonal Frequency Division Multiple Access). In the case of the logic of the shared channel allocation such as the OFDMA wireless technology illustrated in FIG. 2, the resource allocation to a user is not always guaranteed on both of a frequency base and a time base. Hence, there is a possibility in which the user might get into such an event that the user is not provided with want-to-receive resource allocation when required. Further, in a handover process also, it is difficult for the handover source base station and the handover target base station to allocate the same frequency resource at the same timing to the target mobile station, with the result that the handover is carried out basically by a channel switching method.
The handover scheme of the channel switching method, however, entails keeping continuity of the data on the mobile station and preventing a loss of data during the handover process. What is examined for attaining this is a method (which is called a [data forwarding process]) of forwarding the user data for the mobile station to the handover target base station from the handover source base station so that the data directed to the mobile station is not lost to the greatest possible degree when in the handover process.